I have coupons that I printed from the California Fun Spots website:
http://www.cafunspots.com/he_coupons.html
You have to type in your name & address to access the coupons. The print-out I have has:
$8 off one-day general admission to Legoland, limit 4 per coupon
$8 off Best-value admission to San Diego Zoo, limit 6
$7 off Full-price admission to Seaworld, limit 6
$8 off Nairobi Package at Wild ANimal Park, limit 6
$8 off Universal Studios, limit 6
All the coupons expire March 31, 2007 -- but I printed them awhile ago.
I was able to get free Legoland passes using my canadian airmiles. There might be some other similar-US programs that offer them too.